This invention relates generally to solid-state laser arrays and, more particularly, to techniques for achieving a desirable far-field radiation pattern from arrays of this type. It is well known that high output powers can be obtained from arrays of semiconductor lasers if the individual lasers can be phase locked. Ideally, a phase-locked laser array should produce a coherent optical beam having an extremely narrow beam width that is limited only by diffraction effects, known as a diffraction-limited beam. Moreover, the ideal far-field distribution pattern is single-lobed.
Most laser arrays exhibit multi-lobed far-field radiation patterns, with divergence angles in excess of the diffraction limit. A number of different approaches have been proposed to achieve the desired single-lobed and diffraction-limited beam, usually involving modification of the array structure or of the simple lasing action of the semiconductor lasers making up the array. One such method is to control the electrical currents through the laser elements on an individual basis, using separate electrical contacts. This is impractical in many cases, especially if the number of array elements is large.
Other techniques involve the modification of the laser array structure to change the coupling characteristics of adjacent elements. These approaches often require some compromise in the desired output characteristics, such as the output power, the single-lobed nature of the far-field pattern, the angular beam width, and so forth.
Phase-adjustment of lasers formed in gallium arsenide has been proposed, for example in Scifres U.S. Pat. Nos. 4,360,921 and 4,462,658 et al. The structure disclosed in the Scifres et al. patents is monolithic, i.e. a laser is fabricated on the same structure as a waveguide. The Scifres et al. structure includes an array of electrodes employed to produce a phase retardation along the wavefront of a propagating light beam, thereby effecting a desired angular scanning action of the beam, or focusing it as desired. Another application of electrooptically induced phase changes is shown in Yao U.S. Pat. No. 4,285,569 et al. This patent discloses an integrated optical modulator array in which a uniform grating and a chirped grating are used to provide an array of beams, which are then subject to phase adjustment in an electrooptic modulator.
It will be apparent from the foregoing that there is still a need for improvement in the field of semiconductor laser arrays. In particular, there is a need for a simple array structure that can achieve a desirable far-field output pattern, with a predominantly single-lobed and diffraction-limited beam, but without the disadvantages of the prior art. The present invention satisfies this need.